This invention relates to a process and apparatus for operating a firebox for steam generators with solid gasifiable fuel, particularly coal dust having poor kindling properties and a low volatile gas contents.
Such fuels having poor kindling properties and a low contents of volatile gases include the so-called anthracites or lean coals, that is coal having a very low contents of volatile gases of about 2 to 8% relative to the crude coal.
Boilers for power plants are known which are operated with lean coal and are provided with one or several molten ash chambers where accordingly a liquid ash discharge occurs. This liquid discharge caused by the necessary higher firebox temperature results in higher formation of nitrous oxides in the flue gas then would occur with a dry ash discharge. If a firebox is used with a dry ash discharge the firing action must be specially stabilized by supporting fire arrangements.
In a prior art process (accepted West German application No. 1,107,876) it has been proposed to burn the dust-like fuel which has poor ignition properties by heating it prior to the entry into the combustion chamber in several stages until the ignition point of the coal dust is reached. As heating medium there can be used flue gases or preheated air which blows the dust into a preheat device. After the preheat stage the coal dust is separated from the preheating medium.
This process requires complex apparatus for heating the coal dust and separating the preheating medium.
It has also become known in apparatus for coal dust firing (West German Pat. Nos. 4 55 571 and 5 51 238) to burn the dust in two stages in which case the dust in the first stage is converted into a more or less gaseous stage. The introduction of a secondary air can be effected at several places of the combustion chamber.
With all of the prior art there has always been the problem that when lean coal is used as fuel for the firing box the poor ignition properties of the coal compared to coal having a higher portion of volatile components require an increase of the temperature of the combustion air if better ignition properties are wanted. Such increase of the temperature of the combustion air however results in an increased formation of the noxious NO.sub.X compounds (nitrous oxides) in the flue gas.
On the other hand, if a lower temperature in the firing box could be obtained it would be possible to get a discharge in the form of a dry ash. Heretofore lean coal could not be properly ignited at lower temperatures and therefore a dry discharge could not be obtained without additional firing means.
The present invention therefore has the general object to provide for a process and apparatus which assures the ignition of fuel of low ignition properties and low contents of volatile gases, such as anthracite lean coal, and nevertheless to effect the combustion at a temperature where a dry discharge is possible and thus the contents of nitrous oxides in the flue gas can be maintained at a low level.